We explained why this is so difficult: even
under the most intensive current therapy, a silent ‘reservoir’ of a type of CD4
cell called ‘memory cells’ remains infected with HIV. These are like sleeper cells
in a resistance organisation – their job is to spring into action when a
specific infection they are primed to recognise turns up. In other medical
conditions, vaccines work by tricking cells to ‘recognise’ an infection without
actually having had it. The trouble is, when HIV-infected memory cells spring
into action, they start spewing out HIV.

We can flush HIV-infected memory cells out
of hiding by activating them and then kill them: but the burst of HIV they
produce in this process causes more CD4 cells to be infected.

Last month, we explained how Brown’s doctor,
Gero Hütter, got round this by destroying Brown’s CD4 cells and then re-introducing
others, via a bone marrow transplant, from a donor naturally resistant to HIV
(missing the CCR5 co-receptor, which HIV grabs on to). However, a bone marrow
transplant, while the standard second-line treatment for leukaemia, is far too
toxic – and expensive – for general use and indeed nearly killed Brown.

It is, however, proof that a cure is
possible. The most promising approach towards a cure for all is to do at least
one of the two things Dr Hütter did, but in a much more subtle way.

1. Re-engineer CD4 cells

One approach could be to take bone marrow
cells from the patient’s own body, and by means of enzymes and genetic tools,
engineer them to become CCR5-negative, thus protecting them against further HIV
infection. You then re-introduce them into the patient’s body, in a so-called
‘autologous’ – meaning self-donated – transplant.

The hope is that the CCR5-negative cells would
slowly start to take over from the CCR5-positive cells and HIV would slowly be
starved of the cells it needs in order to reproduce.

Even people on effective antiretrovirals
maintain a viral load averaging three copies/ml and this appears to contribute
to keeping the immune system in a permanently higher state of activation than in
HIV-negative people. This activation kills off some HIV-infected cells but
infects others, keeping the reservoir topped up, or so the theory goes. If,
however, a population of infection-proof cells were introduced, they would come
to predominate as there would be fewer cells to infect as time went by.

This approach has actually been trialled
successfully, in mice genetically modified to be susceptible to HIV. Researcher
Paula Cannon and her team from the University
of Southern California
used a drug called SB728, a so-called zinc finger nuclease enzyme, to snip out
CCR5 from mature CD4 cells and then re-introduce them into the blood.2
They then infected these mice and a control group with HIV. The control mice
lost their CD4 cells and developed AIDS within 8 to 12 weeks but the mice given
CCR5-negative cells maintained normal CD4 counts and undetectable HIV viral
loads.

Many scientists are sceptical that the reservoir
of HIV-infected cells could be replaced by HIV-proof cells unaided. The CCR5
cells in Cannon’s mice were by no means eliminated, especially the
all-important progenitor cells in the bone marrow. Steven Deeks, a prominent
cure researcher from the University
of California, San
Francisco, says: “They did the transplant first and then the infection.” It
might not work in people already infected, where there is an established
reservoir of HIV-infected cells.

Even if it does work, it could take a long time
for one cell population to replace another: “In mice it happens in months, in
people it could take years,” Deeks told HTU.

Nonetheless Cannon and her colleague John
Zaia are now leading a Phase I trial in patients with lymphoma, using bone
marrow transplants of patients’ own genetically engineered progenitor cells to
try to ensure the growth of a CCR5-negative cell population.3

2. Delete infected cells

Alternatively, one approach could be to concentrate
more on the immune-destruction part of Timothy Ray Brown’s therapy instead of
the CCR5-deletion bit. The idea would not be to crudely annihilate all the
cells HIV might infect. Instead we could:

‘Purge’.
This strategy involves enticing reservoir cells out of
hiding using drugs that ‘switch on’ reservoir cells so they become activated
and therefore detectable, while keeping patients on antiretrovirals so that the
activated cells do not go on to seed new infection. The HIV-infected activated
cells would then destroy themselves, and the idea is that repeated cycles of
activation would deplete the reservoir beyond the point at which it can
replenish HIV – a strategy that’s been called ‘purge’.

Experiments were
done more than five years ago using the drug valproic acid (Depakote). This is a member of a class
of drugs called HDAC inhibitors, which take the genetic brakes off resting
cells. In one study, three out of four subjects given valproic acid achieved a
70% reduction in the number of HIV-infected reservoir cells.4 It
appears, however, that this reduction may only be temporary: two larger studies
in 2008 showed no long-term reduction in the number of HIV-infected reservoir
cells in other patients.5,6

This may be
because valproic acid is not strong enough. Trials are planned of a stronger
HDAC inhibitor called vorinostat (Zolinza),
a cancer drug already used for some types of lymphoma and which is being
trialled for anal cancer.7 “Vorinostat is a tremendously powerful
drug,” says Deeks.

If HDAC inhibitors
turn out not to work, there is a second family of drugs called HMT inhibitors,
some of them already in use as cancer drugs, that reawaken latently infected
cells in a different way. They are only just starting to be studied.8

‘Kill’.
We don’t yet know if activating HIV-infected cells would
cause so many to commit cellular suicide that HIV would be purged from the
body. Instead of enticing cells out of hiding by activating them and seeing if
they blow themselves up, how about a more aggressive strategy of directly
seeking them out and killing them in their sanctuary sites? Amazingly, attempts
to do this date from as long ago as 1988, when a group devised a drug ‘missile’
that combined an antibody that locked on to the CD4 molecule with a
cell-killing toxin derived from the pneumonia bacterium Pseudomonas. It wasn’t taken further because it wasn’t selective
enough, targeting all CD4 cells.9

By 2002, we were
able to make more specific antibodies that only locked on to the memory cells
that form the reservoir, and a team devised a similar cell-missile that
eliminated a proportion of latently HIV-infected cells in the test tube, from
blood taken from patients with HIV. The trouble is that while it cut the number
of HIV-infected reservoir cells by at least 80%, it probably didn’t eliminate
enough, while at the same time picking off rather a lot of non-infected memory
cells.10

‘Shock
and kill’. We still don’t have a way of infallibly
identifying only those one-in-a-million memory cells latently infected with
HIV, so we can’t kill them and only them. So researchers are devising
combination drug missiles that would both entice HIV-infected cells out of
hiding and then seek them out actively for destruction. The idea is to devise a
three-component therapy that would combine an immune stimulant, an antibody
that seeks out activated cells, and a toxin to destroy the targeted cell, a
strategy that’s been called ‘shock and kill’.

One of the
possible problems with both ‘purge’ and ‘shock and kill’ is that anything
strong enough to activate enough immune cells might be too toxic to use – as
has already proved to be the case with drugs like IL-2. In particular, some
researchers are concerned that it may cause inflammation in places like the
brain which may have been what happened to Timothy Ray Brown: an opinion piece
warning about this appeared recently in the journal AIDS, recommending that attempts to deplete the reservoir this way
should be started gradually.11

What we really
need is a drug that stops cells from being ‘latent’ and gets them to rejoin the
actively circulating, and therefore visible and vulnerable, force of T-cells
without widespread immune activation. Researcher Robert Siliciano and his team
at Johns Hopkins
University in Baltimore are involved in identifying small
molecules that could manage this feat, gently teasing the immune cells out of
hiding instead of shocking them, and in 2009 identified the first one, a
compound called 5HN.12

3. Delete resting cells

Another strategy
is to try and find markers that uniquely identify infected reservoir cells
while they are still resting, and kill them without ever having to activate
them. Just because we have found no such markers yet does not mean they don’t
exist. Researcher Rafick-Pierre Sékaly, scientific director of the recently established
Vaccine and Gene Therapy Institute of Florida, is investigating possible
chemical markers, including an enzyme called PDI (protein disulfide isomerase),
which might betray the location of resting HIV-infected cells. Sékaly has
identified a multiplicity of active genes that characterise resting cells and
appear to keep them quiescent, and has also discovered that the presence of
another kind of cell called myeloid dendritic cells may be necessary to keep
them that way.13

Equally, HIV may gravitate towards cells that
display particular kinds of biomarkers already, other than the ones we already
know, and we could become able to characterise the subset of cells that is most
likely to become infected with HIV and target just those for destruction. The
cellular receptors CCR4 and CXCR3 have already been found to characterise
immune cells in the gut that are more likely to become infected.14

4. Dry up the reservoir

Cells don’t just passively stop producing
HIV and go into quiescent mode by themselves. The process through which a small
minority of CD4 cells join the reservoir of resting memory cells is controlled
by a complex chemical pathway whereby specific genes are turned off – just like
the lights at bedtime. Instead of trying to prod the resting cells to come out
of hiding, we could keep these genes active and stop them ever going into
hiding in the first place. Protein disulfide isomerase (PDI) is in a family of
enzymes that seem to be involved in this process, but there are many more.

One old favourite is a molecule called
nuclear factor kappa B (NFκB), a ubiquitous gene activator that
was first investigated as a possible target for HIV drugs 20 years ago. Aspirin
is a NFκB inhibitor, though its effect is far
too weak and non-specific for HIV therapy. Low levels of NFκB
are generated during the low-level viral replication seen in antiretroviral
therapy, and these levels appear to help keep the HIV reservoir replenished. If
you could find a drug that had a much more specific effect on NFκB
or one of the other molecules in the cell suppression/activation pathway, you
might be able to stop cells joining the reservoir. Conversely, if you stimulate
NFκB or related molecules with a stimulant drug
like the plant derivative prostratin,15 you turn reservoir cells
into activated ones – another example of the ‘purge’ approach.

However, that also illustrates a problem:
some of these cellular proteins, like NFκB, do tremendously complex cellular
jobs containing many feedback loops. In one situation they are activators, in
another, suppressors, and you may find that inhibiting them has the opposite
effect to the one you want. Scientists are therefore investigating drugs that
inhibit the mechanism whereby the reservoir gets replenished in other ways.
Amongst these is a drug called hexamethylene bisacetamide (HMBA) which might be
able to stimulate HIV-infected reservoir cells without activating non-infected
ones.16

Prostratin is quite an exciting drug.This is because, while it stimulates
cells to come out of hiding and therefore makes them vulnerable to self-destruction
or attack, it also ‘downregulates’ the CCR5 receptor, and indeed another
receptor called CXCR4 which some types of HIV use to get into cells. This means
that it could be our best shot yet at a drug that purges infected cells but
makes other cells less likely to be infected. Prostratin itself looks rather
toxic and until recently, has only been available as an expensive extract from
the bark of a tree from Samoa, where it has
been used to treat liver disease for centuries. Scientists have recently
discovered how to make a cheap synthetic version, which means they can start
doing bulk searches of similar molecules to find less toxic drugs of the same
type.17

The 'combo' cure

We’re used to combination therapy against
HIV and have more recently started talking about combination prevention. A cure
for HIV is also unlikely to involve one ‘magic bullet’. Any cure is likely to
involve several different approaches, used together or sequentially.

For instance, we don’t yet know if there is
a threshold number of infected cells below which active HIV replication is very
unlikely to restart. It’s like cancer: can we tolerate a few infected cells in
the body, or will the presence of even one eventually lead to the return of
HIV?

We could therefore use HDAC and NFκB
inhibitors to flush out the majority of infected cells, use engineered CCR5-negative
cells to try and replace them, and use a therapeutic vaccine to mount continued
surveillance against whatever small minority of HIV-infected cells might still
remain. Or – since one of the problems with therapeutic vaccination is that it
depends on enhancing an immune response, which may lead to more infection – use
an immune-suppressant drug to ‘lock down’ the infected remainder.

There have been a number of attempts already
to deliver several HIV eliminators in one package. For instance, the Australian
biotech company Benitec has devised a combination consisting of an enzyme that
snips out CCR5 from CD4 cells, combined with sections of ‘interfering’ RNA that
delete HIV’s reverse transcriptase enzyme and its Tat protein, the viral toxin
that over-excites CD4 cells into an HIV-receptive state in the first place.
This is all wrapped up in a vector, the shell of an HIV-like virus that infects
cells with the genetic products and gets them to start making them. Zaia’s team
at the City of Hope Hospital in Duarte,
California, has already done a Phase
I proof-of-concept trial in lymphoma patients in which the genetically modified
cells produced the HIV-disabling products for over two years, though only at
low levels.18

We are only as yet on the first steps of a
journey towards making a cure practicable for all, though in researching this
article I sensed a new confidence amongst researchers that it might be
possible. Many refused to guess at timelines, but Steven Deeks told me that a
usable cure strategy would take “at least ten years”.

Sharon Lewin of Monash
University in Melbourne,
Australia, made a keynote address
at the opening of the International AIDS Conference in Vienna last year,19 and, with
Nobel Laureate and co-discoverer of HIV, Françoise Barré-Sinoussi, was
instrumental in pulling together a pre-conference two-day workshop on strategies
towards a cure.20

In her keynote address she said she was
encouraged by two major cure-research initiatives now underway: amfAR’s ARCHE
initiative, which had a budget of $1m, and the Martin Delaney Collaboratory, a
public/private partnership of research labs funded to the tune of $8.5 million
by the US National Institutes of Health and named after the late AIDS activist
who founded Project Inform. However, she pointed out that less than 10% of the
current funding for an HIV preventive vaccine is currently devoted to curing
HIV.

“Cure research doesn’t have to be hugely
expensive,” she told HTU. “You don’t
need the big trials with tens of thousands of people you need for vaccine and
biomedical prevention studies. The initial discoveries can be made with studies
of 100 people. But we do need large, multidisciplinary consortia like the
Martin Delaney project to ensure that research is co-ordinated and not
wasteful.”

The final question, though, is one only
Deeks addressed, among the researchers I talked to. We can control HIV and the
illness caused by it, but it’s becoming apparent we may never be able to treat
everyone because of the massive levels of funding, human resources and healthcare
provision needed. Will the same be true of a cure?

“A cure is going to be expensive,” he said.
“If we were going to do it with aspirin we’d have done it by now. It may also
carry with it a degree of risk, and researchers and patients may have to ask
themselves how much risk they are prepared to tolerate if the result is going
to be elimination of HIV.

“But it’s going to be a lot more affordable
than lifelong antiretrovirals in resource-rich countries. As to whether it
would be scalable for poor countries, though – ah, that’s a very different
question.”

Biancotto A et al. Dual
role of prostratin in inhibition of infection and reactivation of human
immunodeficiency virus from latency in primary blood lymphocytes and lymphoid
tissue. J Virol 78:10507–15, 2004.

Issue 204: March 2011

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member of your healthcare team for advice tailored to your situation.

The Community Consensus Statement is a joint initiative of AVAC, EATG, MSMGF, GNP+, HIV i-Base, the International HIV/AIDS Alliance, ITPC and NAM/aidsmap

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This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends
checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member
of your healthcare team for advice tailored to your situation.